Honestly, it looks like a serious auto-conversion error. I have no idea why your book is bolding all over like that, but I'm pretty sure it's not supposed to be. Likely someone started out with a shoddy source file and fed it into Calibre, which can only try its best with what it's given.
Since the bolding doesn't seem to do anything useful, I'd just get rid of it by doing a find/replace for
</b> <b class="calibre3">
and replace it with a single space, then replace
<p class="calibre2"><b class="calibre3">
and
</b></p>
with the plain, <p class="whatever">-only versions of the markup.
It's how I clean the cruft from my own e-books when I decide to redo the formatting to my liking, and usually a lot faster and easier than trying to build a new book from scratch using cut-and-pasted text.
Since you're converting from ePub to Mobi, I'd say when in doubt, discard or at least comment out the parts of the CSS file you don't understand, which don't seem to do anything useful.
Often Calibre conversions put in a lot of redundant stuff which you just don't need and can't use, given Mobi's limited display capabilities.
J'espère que cela vous aide, et bienvenue à MobileRead!